Method for bonding sheet metal cladding to a body

ABSTRACT

A method for applying a cladding to the surface of a body by enclosing the body with a cladding of sheet metal or foil loosely adherent to the body surface in a mass of a non-reactive, nondensifying powder, and compressing the powder at a pressure of greater than about 500 psi and at elevated temperatures up to the melting point of the body or the cladding.

United States Patent Lange 1 Oct. 24, 1972 [54] METHOD FOR BONDING SHEET3,114,961 12/1963 Chambers et a1. ....29/ 156.8 B METAL CLADDING TO ABODY 3,141,227 7/ 1964 Klepfer et a]. ..29/497.5 X 3,235,958 2/1966Southron ..29/497.5 X [72] Invent Frederick Lang" 3,037,883 6/1962Wachtell et al ..29/194 x [73] Assignee: Westinghouse ElectricCorporation, 3,469,062 9/1969 Berkley ..29/492 X Pittsburgh, Pa. PrimaryExaminer-John F campbell [22] F'led: 1971 Assistant Examiner-Ronald J.Shore [21] App1,N 132,378 Attorney-1 Shapoe and L. P. Johns [57]ABSTRACT [52] US. Cl ..29/497.5 51] 1111. C1. ..B23s 31/02 A method p gs a q s to of a h ..29 497.5, 493, 156.8 B by m 08mg 1 e body a Clad mg0 Sheet [58] Field of Sean I metal or foil loosely adherent to the bodysurface in a [56] References Cited mass of a non-reactive,non-densifying powder, and compressing the powder at a pressure ofgreater than UNITED STATES PATENTS about 500 psi and at elevatedtemperatures up to the 1t t fth bod th ladd' 1,081,618 12/1913 Madden..29/421 UX me mg pom 6 y e c 2,932,882 4/1960 Kelly, Jr ..29/421 X 5Claims, 2 Drawing Figures CROSS REFERENCE TO RELATED APPLICATION Thisapplication is related to pending application Ser. No. l35,642,filedApr. 20, 1971.

BACKGROUND OF THE INVENTION 1. Field of the Invention This inventionrelates to a method for applying metal cladding to a body and, moreparticularly, it pertains to the application of foil or sheet claddingto a body having a complex configuration.

2. DESCRIPTION OF THE PRIOR ART Many techniques exist for cladding metalcomponents with sheet metal to enable the components to resist acorrosive environment. For example,,cladding of gas-turbine blades witha more corrosive resistant metal, such as Nichrome, not only increasesthe life of the blade but also allows higher operating temperatures andtherefore more eff cient gas-turbine operation. The bonding of sheetmetal as cladding'to complex shapes such as gas-turbine blades, isconsiderably more difficult than techniques required to apply suchcladding to flat metal surfaces. Inasmuch as gas-turbineblades are usedin conditions involving high velocity, high temperature,'and corrosivegasstreams with large radical stresses, the bonding mustnot only bereliable with regard to corrosion, it must also be mechanically strong.Such a bond between the cladding and the base metal object is bestobtained bydiffusion bonding; that is, diffusion of atoms of thecladding material and of the base metal object into each other to form acoherent bond between the cladding material and the base metal object. I

In addition, diffusion bonding requires intimate contact between thecladding surface and the base metal surface. For example, for simpleshapes such as flat objects for discs, an adequate pressure must beapplied at an elevated temperature to promote such diffusion. That isessentially the technique in forming sheet metal stock for silver cladcoins and the like. For more complex shapes, such as pre-cast air-foilshapes, the same requirements must be satisfied; there must be intimatecontact obtained by pressure and high temperatures for diffusion.

SUMMARY OF THE INVENTION In accordance with this invention is has beenfound that the problem of applying a metal cladding to a metal objecthaving a complex configuration is satisfied by the steps ofpreliminarily attaching a metal cladding to the surface of a metalobject to be covered, inserting the assembly of the cladding and metalobject into a mass of a powder vehicle comprising sufficient volume tocompletely enclose the assembly within the chamber of a pressure mold,and applying pressure and heat either simultaneously or otherwise to thepowder vehicle in order to press the cladding against all surfaces ofthe metal object for a sufficient time to diffusion-bond the cladding tothe surface of the object.

This method of diffusion-bonding eliminates the prior disadvantages ofhot-gas isostatic pressing methods and enables diffusion-bonding ofmetal cladding to most all components of complex configuration.

2 BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic view of an airturbine blade and a sheet metal cladding in position prior to placementon the surface of the blade; and

FIG. 2 is a vertical sectional view through a hot pressed furnace inwhich the sheet metal cladding is diffusion-bonded to the surface of theturbine blade.

DESCRIPTION OF THE PREFERRED EMBODIMENT The method of bondingsheet'metal cladding to a body having an irregular or complex surface inaccordance with this invention is exemplified by describing the mannerin which a. gas turbine blade is covered with a sheet metal cladding. InFIG. 1 a gas turbine blade, generally indicated at 1, includes a base 2and a fin or airfoil 3. The airfoil which extends outwardly mallycomposed of a metal or a highly densified metal oxide. In operation, ithas been found that the arcuate surface 5 is usually susceptible toexcessive corrosion and abrasion due to the impact of hot gases upon thesurface. of the blade. For that reason, it is desirable to provide acover or protective cladding on the blade surface and to extend theblade life by minimizing the corrosion thereof by the steam.

As shown in FIG. 1, the fin 3 of the blade may be provided with a sheetmetal clad 8 by pre-forming a portion of metal sheet or foil of asuitable corrosion resistant material such as an alloy containing aboutpercent nickel and 20 percent chromium and having a thickness of about 3mils. The clad 8 is pre-formed on a suitable mold after which it isplaced on the fin 3 by sliding it downwardly in the direction of thearrows 9 until it completely covers the fin. The blade 1 is then readyfor the diffusion-bonding.

Generally, the method of this invention includes the steps of:

1. Pre-forming a cladding of sheet metal for loose attachment to a basemember;

2. Placing the pre-form on the base member;

3. Inserting the assembly of the base member and the pre-form into apressurizing chamber and enclosed therein within a mass of a powdervehicle;

4. Compressing the powder vehicle at a pressure (necessary to causeintimate contact) of from about 500 to 5,000 psi to press the looselyfitting I pre-form against the surfaces of the tin; and, 5. Heating theassembly to a temperature below the melting point of the base object andthe cladding, and holding the assembly at the indicated pressure andtemperature for a time sufficient to bond the nickel and 20 percentchromium, has a thickness of from about one-half to 15 mils andpreferably about 3 mils. The surfaces of the pre-formed clad 8 and fin 3are preferably cleaned to remove any adherent dirt or oxide immediatelyprior to their assembly.

Before the assembled clad and fin are inserted into the pressure vesseland furnace they are preferably secured together by the use of adhesivetape or spot welding in order to prevent particles of the powder vehiclefrom entering any spaces between the cladding and the base duringplacement of the blade into the furnace.

As shown in FIG. 2 the blade 1 is placed into a furnace generallyindicated at 10 within a chamber 11 of a cylindrical -mold 12 whichchamber is filled with a powder vehicle 13. The lower end of the chamber11 is closed by a block or piston 14 and the upper end is similarlyclosed by a block or piston 15. After the blade l'is completely enclosedwithin the powder vehicle 13, pressure is applied to the powder vehicleby the application of pressure through one or both pistons 14 and 15 asshown by the arrows 16. The mold 12 may be composed of graphite wherepressures below 3,000 psi are to be applied. Where greater pressures areused, the die is preferably composed of metal such as Nichrome ormolybdenum alloy. The mold is completely enclosed in an insulationmaterial 17 which in turn is encased within a quartz cylinder 18. Meansfor heating the furnace, such as an induction coil 19, are providedexternally of the cylinder 18.

The powder vehicle 13 is composed of particles of any material that isnon-reactive with the metal parts; namely, the base 1 and the clad 8.For example, the powder vehicle may be composed of particles of SiC, A10 ZrO TiO MgO, BN, BeO, NaCl and other metal oxides such as SnO whenused below about l,O00 C. In addition, other materials may be usedincluding oxides, carbides, nitrides, borides and silicides of metals,such as single oxides, mixed oxides, intermetallics, and refractorymetal powders. The powdered material should be compressible butnon-densifying so that it is readily removed from the metal parts afterthe pressure operation is completed and the assembly removed from thefurnace.

In operation, a pressure of up to about 3,000 psi is applied to thepowder vehicle 13 through one or both of the pistons 14 and 15. Theassembly is then heated to a temperature of about l,O00 C where it isheld for a sufficient time such as about 1 hour for diffusion-bonding tocommence. Thereafter, the furnace is cooled to room temperature and theassembly removed. During the pressing and heating operations, thefurnace atmosphere is preferably a vacuum although a mixture of aforminggas composed of nitrogen and hydrogen may be used. Better bondingstrengths are obtained, however, by the use of a vacuum atmosphere.

The pressure applied at various temperatures is dependent upon thethickness of the foil. Where very thin foils such as less then 3 milsand about one-half mil are used, lower pressures such as below 1,000 psiare preferred. However, where cladding having thicknesses of greaterthan 3 mils, higher pressures such asat least 1,000 psi and up to about5,000 psi may be necessary.

Although better bonding between the members is obtained with longertimes at temperature in the furnace,

it has been found that once diffusion begins it continues upon removalof the pressure. Improved diffusionbonding conditions may be achieved byplacing the cladded blade in a hydrogen atmosphere at about l,O00Cfor 24hours.

The following example is exemplary of the invention:

EXAMPLE A gas turbine blade similar to that shown in FIG. 1 and having atotal height of 3 inches is provided with a preformed sheet metal cladby placement of the clad on the fin portion of the blade. The blade iscomposed of Udimet 500 and the clad is composed of Nichrome V and has athickness of 3 mils. The loose edges and top end of the cladding aresecured together by a pressure adhesive plastic tape to preventparticles of the powder chamber is then filled with silicon carbidepowder with the powder vehicle completely enclosing the blade. The upperpiston 15 is then inserted in place and a preliminary pressure of 3,000psi is applied to the powder which pressure is transmitted to allsurfaces of the cladding and against the outer surfaces of the fin. Theinduction coil heats the furnace to a temperature of l,O00 C and is heldfor a period of 1 hour after which it is cooled to room temperature inthe furnace. As a result of the foregoing procedure a cladded fin for agas turbine blade is provided having a satisfactory degree of thermaldiffusion for providing corrosion resistance to the base material of thefin during its use as a turbine blade.

In this manner, any complex shape can be cladded by diffusion bonding.

What is claimed is:

l. A method for diffusion bonding sheet metal cladding to a base membercomprising the steps of mounting a pre-fonned sheet metal cladding inplace on a metal base member, placing the cladded base member into apressure chamber, enclosing the cladded base member in a powder of anon-reactive material selected from a group consisting of a metal oxide, metal carbide, metal nitrides, and metal borides, applying apressure of at least 500 psi to the powder to isostatically transmit thepressure uniformly to the cladding and against the surfaces of the base,heating the assembly of the cladded base member and powder to anelevated temperature at least that at which diffusion takes placebetween contacting surfaces in a reasonable time period but less thanthe melting points of the metal cladding and the base member, andholding the pressure and temperature conditions for a sufficient time todiffusion bond at least a portion of the cladding and base together. 2.The method of claim 1 in which the pressure is applied from about 1,000psi to about 5,000 psi.

3. The method of claim 1 in which the cladded base member is heated at atemperature of from about 600 C to about l,O00 C.

4. The method of claim 1 in which the pressure transmitting powderis atleast one of the materials selected from a group consisting of SiC, A1TiO ZrO MgO, BN, BeO, NaCl, and Si N 5. The method of claim 1 in whichthe powder is a 5 non-reactive, non-densifying metal oxide.

2. The method of claim 1 in which the pressure is applied from about1,000 psi to about 5,000 psi.
 3. The method of claim 1 in which thecladded base member is heated at a temperature of from about 600* C toabout 1,000* C.
 4. The method of claim 1 in which the pressuretransmitting powder is at least one of the materials selected from agroup consisting of SiC, Al2O3, TiO2, ZrO2, MgO, BN, BeO, NaCl, andSi3N4.
 5. The method of claim 1 in which the powder is a non-reactive,non-densifying metal oxide.